Hoggy, after reading your second paragraph above, I get a little confused. All I know is that whatever you do in LR is nondestructive, so whether you edit the original image or not, it is exporting (saving) the file that compresses the JPEG.
When setting the option 'Original' in the export dialog, it's the only exception in which it doesn't [re]compress the image data. It's basically a file-system type copy, in which there is no compression or recompression involved. The only possible change could be in the metadata, which if you've set the option to write metadata into the original jpeg XMP, will actually write it into the original. The original's
image data will never be changed though. XMP is something that may 'live' within some file types - it may be in the same physical file, but it's separate from any image data.
Of course, when using that 'original' setting from the dropdown, no edits from
Develop module will show up - because it's for all intents and purposes, a file-system type copy.
Cletus mentioned converting JPEG into TIFF first. But when I export it from Print module, doesn't it get changed back to JPEG again? Besides, my lab accepts only JPEGs.
What I think Cletus is referring to, is if you take the original 1st-generation jpeg and convert(export) it to a TIFF - and then, for instance, re-import that TIFF into LR and make image edits, any further jpeg exports from that point on
from that TIFF will always be 2nd-gen jpeg. IOW, the TIFF might be likened to a raw file at the point it's made. As far as I know, there is no lossy compression for TIFF's - certainly not in LR there isn't.
As for exporting from the Print module - I've never actually printed anything.. But looking at the Print Module, I don't see an export option. So either you're referring to 'printing to a file' in the Print dialog that would pop up after pressing 'Print/Printer', or you're actually choosing "Export" from the "File" menu in the top menu-bar - which is actually the standard Export dialog that would show up from any module.